1. Field of the Invention
This invention relates generally to a process for forming an improved superconductor/semiconductor junction structure, such as a Josephson junction device or a super-Schottky device, which has an interfacial reaction barrier, and to structures formed by such a process; and, more particularly, to a process for forming such a structure having silicon as the semiconductor and niobium as the first superconductor material, with a layer of second chosen superconducting material formed between the niobium and silicon.
2. Description of the Prior Art
Superconducting devices, such as Josephson junction devices and super-Schottky devices, are finding increased use in computer electronic circuits where fast switching speeds and low power dissipation levels are required and also in millimeter wave and microwave detection circuits and mixers where there are low noise requirements.
A Josephson junction superconducting structure of the tunnel-junction type is described in the publication entitled "Josephson Tunnel-Junction Electrode Materials," by C. J. Kircher and M. Murakami, in SCIENCE, Vol. 208, 23 May 1980, pages 944-950, and further in the publication entitled "The Superconducting Computer", by J. Matisoo, in Scientific American, Vol. 242, No. 5, May 1980, pages 50-65. Such a Josephson junction superconducting device may be composed of several superconducting tunneling junctions having shared common electrodes as shown in the publication by Kircher et al. The junction portion of the structure consists of two superconducting electrodes on either side of an ultra-thin insulating layer, which is sufficiently thin that super-current (i.e., zero voltage current) can pass through it by electron pair tunneling. The electrodes are formed of a superconductor material, that is, a metal that has an infinitesimally small electrical resistivity (i.e., zero resistivity) when cooled to below a characteristic temperature T.sub.c, typically 20.degree. K. or lower. In the prior art, the Josephson device has been fabricated using an oxide layer as the insulating layer which serves as a tunneling barrier.
However, we have recently shown that superconducting Josephson junction devices can be fabricated using an ultra-thin silicon membrane as the tunneling barrier, and niobium (Nb) as the superconductor electrode, as described in the publication entitled "Super-Schottky and Josephson Effect Devices Using Niobium on Thin Silicon Membranes," by Lynette B. Roth, John A. Roth, and Paul M. Schwartz, in Future Trends in Superconducting Microelectronics, American Institute of Physics Proceedings, Vol. 44, edited by B. S. Deaver, C. M. Falco, J. H. Harris, and S. A. Wolf, New York, 1978, pages 384-388. The use of a silicon substrate rather than an oxide layer as the tunneling barrier in a Josephson junction device has the advantage that the silicon barrier can be 10 to 100 times thicker than the equivalent oxide barrier, which permits greater ease of fabrication. Moreover, all the capabilities of the highly developed processing technology for silicon may be used. In addition, the use of silicon barriers in Josephson junction devices permits the operating characterics of these devices to be optimized for each particular application by modifying the doping profile within the silicon.
A super-Schottky device is somewhat similar in structure to the Josephson junction superconducting device discussed above, in that a super-Schottky device also has a superconductor/semiconductor junction. The structure and function of a super-Schottky device is described, for example, in the publication by F. L. Vernon, Jr., M. F. Millea, M. F. Bottjer, A. J. Silver, R. J. Pedersen, and M. McColl, IEEE MTT-25,286 (1977). Basically, a super-Schottky device consists of a semiconductor layer, such as silicon, on one surface of which is formed a superconductor layer, such as niobium, to provide an interface therebetween which is the site of the Schottky barrier. On the opposite surface of the semiconductor layer there is formed a layer of a conventional metal, such as aluminum.
In both the super-Schottky device and the super-conducting Josephson junction devices, in which there are superconductor/semiconductor junctions, and which are thus referred to herein as superconductor/semiconductor junction structures, device performance is highly sensitive to the precise electronic structure of the superconductor/semiconductor interface since the low temperature current transport in these two types of devices occurs by super-conductive quasi-particle or electron pair tunneling, respectively. Specifically, the presence of non-superconducting or low T.sub.c compounds at the superconductor/semiconductor interface results in spurious electronic states that permit unwanted currents to flow in the device. For example, such excess currents degrade (i.e., decrease) the voltage-state resistance of Josephson junction logic gates and increase the noise figure of super-Schottky mixers. Therefore, the successful development of super-conducting devices using Nb/Si or other superconductor/semiconductor junctions requires certain control over the nature and extent of interfacial chemical reactions which may alter the electronic structure in these systems. This control is especially important for certain transition-metals, such as niobium or vanadium, which are known to readily form silicides when in intimate contact with silicon.
Consequently, we have studied the superconductor/semiconductor interface of selected transition metals that can potentially have interfacial reactions with silicon in order to establish the extent and morphology of interfacial compound formation. As described in our publication entitled "Interface Structure and Electrical Behavior of Nb/Si Junctions," by John A. Roth and Lynette B. Roth, in the Electrochemical Society Proceedings of the Symposium on Thin Film Interfaces and Interactions, edited by John E. E. Baglin and John M. Poate, Vol. 80-2, (1980) pages 111-121, we found that a laterally non-uniform niobium silicide (NbSi.sub.2) layer, having an average thickness of 15 angstroms, forms without intentional heating in Nb/Si samples prepared by deposition of niobium onto atomically clean silicon under ultra-high vacuum, as determined from Auger spectroscopy line shape analysis. Some silicide formation can be expected in any clean transition-metal/Si interface since the heat of condensation of the deposited metal will generally be sufficient to overcome any reaction barriers. In particular, in the Nb/Si system, silicides nucleate and grow during metal deposition, particularly when an atomically clean substrate surface is provided. Such is the case in the ultra-high vacuum deposition of niobium, as employed in some prior art processes in order to achieve good junction formation. The disadvantage of these interfacial reactions is that the compounds so formed are usually non-superconductor materials or low T.sub.c superconductor materials at best, and thus they degrade the superconducting properties and consequent performance of the device, as previously discussed herein.
It is the alleviation of this problem of the formation of unwanted interfacial reaction products in superconductor/semiconductor junction devices to which the present invention is directed. To the best of our knowledge, we believe that we are the first to recognize the problem caused by these interfacial reactions and to provide a solution thereto.